Low-power satellite-based geopositioning system

ABSTRACT

A Low Earth Orbiting satellite system provides location and data communications to mobile users equipped with a receiver/transmitter, which acts as a transponder that responds to a query transmitted over the satellite network. The response is sent after a precisely controlled time interval after the transponder receives the query so that the ground station can estimate the propagation path length. The transponder also transmits the response at a frequency proportional to the frequency of the received query so that the ground station can estimate the first and second derivatives of the propagation path length according to the measured Doppler shift. The ground station also estimates the satellite positioning using satellite telemetry obtained from the on-board GPS receiver. The position of the user terminal relative to the satellite position is then determined from the path length measurements. Given the satellite position and velocity, the measured path length and first and second derivatives determine the angle between the direction of satellite motion and the line of bearing to the user terminal. This angle defines a cone with the satellite at the origin. The user terminal position is somewhere on the circle defined by the cone and the estimated path length. The intersection of this circle with the surface of the Earth yields two possible user positions, which ambiguity can be resolved by three techniques: (1) knowledge of which beam the signal was received in; (2) previously obtained position data; or (3) using nearby satellites to receive the signal.

BACKGROUND OF THE INVENTION

The present invention relates generally to geopositioning systems, andmore particularly to a low earth orbiting satellite-based geopositioningsystem.

This application is related to U.S. patent application No. 08/877,571,entitled "Method and Apparatus for Precision Geolocation," filed on Jun.17, 1997 by Matthew Schor, and assigned to the same assignee of thepresent application. U.S. patent application Ser. No. 08/877,571 ishereby incorporated by reference as if repeated herein in its entirety,including the drawings. This application discloses a technique forimproving a positional accuracy of a satellite-based geopositioningsystem by first measuring the position of a reference at a knownlocation, determining an error vector from the measured position and theknown position, measuring the position of the unknown transceiver, andthen applying the error vector to the measured position. As part of themeasurement process, this application discloses the use of the Dopplershift in the transmitted signal to determine the position of atransceiver. In this system, the transceiver receives a signal from thesatellite and transmits a response. In other words, the transceiver actsas a transponder.

A first requirement of a geopositioning system is that it be applicableto a broad range of uses. To do so, the geopositioning system must beable to determine the location within buildings as well as outsidebuildings. This requires operating with extremely low signal levels dueto the large attenuation caused by buildings. Consequently, mostexisting geopositioning systems are limited in their applicability dueto their inability to receive signals from within buildings.

Another requirement of a geopositioning system is transmission security.Many potential users of geopositioning systems do not necessarily wanttheir locations being broadcast in a way that makes their positionavailable to the public at large. Consequently, the link between thesatellite and the transmitter/receiver should be a low power andrelatively secure transmission.

Spread spectrum communication systems are known to provide thiscapability because they transmit across a broad frequency spectrum andeach frequency cell contains a small amount of transmitted energy. As aresult, the radiated signal of a spread spectrum signal resembles noise.Furthermore, to receive and decode a spread spectrum signal, one mustknow the exact coding used to spread the transmitted signal.

To properly receive a spread spectrum signal, however, one must firstobtain the timing of the transmitted signal. This involves determiningthe frequency as well as the phase of the chipping sequence.

Some known spread spectrum systems transmit at a high data rate, whichenables the receiver to integrate over a short period of time, thusallowing for more frequency uncertainty. However, as discussed above ageopositioning system must be able to penetrate buildings. Consequently,to operate with extremely low power signals, a lower data rate must beused to provide sufficient processing gain to overcome the buildingattenuation. However, longer integration times incur greater losses dueto frequency errors, which means that the time required to acquire thesignal increases.

There are two separate issues with Doppler frequency. The first isgeopositioning accuracy. The transponder must accurately track theDoppler frequency, and the ground station must accurately measure it toprovide good position estimates, as the position estimates are afunction of the Doppler frequency.

The second issue regarding Doppler is signal acquisition. Thetransponder and the ground station initially have only a rough idea ofthe frequency of the received signal because of Doppler uncertainty,which can be tens of kilohertz. If the receiver tunes to a frequencythat is too far from the correct frequency, then the process ofintegration fails to detect the signal due to losses. The maximumacceptable frequency error depends upon the integration time. Longerintegrations require lower frequency errors. If the total Doppleruncertainty exceeds the maximum acceptable frequency error for a givenintegration interval, then the receiver must perform a search for thecorrect frequency. The receiver tunes to a frequency, integrates, andlooks for signal presence. If no signal is detected, the receiver tunesto a new frequency and the process is repeated. The number of times thisprocess is repeated depends upon the integration interval, with longerintegration times leading to longer frequency searches. The receiverdoes not need to know the Doppler within a few Hertz in this case unlessthe integration time is very long.

Thus, the two major problems facing the designer of a geopositioningwaveform are acquisition of timing and Doppler. If the integration timerequired to detect a ranging pulse is T and the chipping rate is f_(c),then a simple serial search of all possible timing offsets using aconventional correlation receiver can take up to T² f_(c) seconds toacquire the ranging signal. For example, with T=1/50 seconds, and f_(c)=1 MHZ, the acquisition time might be as large as 400 seconds, which isalmost seven minutes.

This problem is made even worse if the Doppler offset frequency isunknown as well. The loss L in decibels (dB) due to a coherentintegration across T seconds with uncompensated Doppler of f Hz is givenby: ##EQU1## This loss is shown as a function of the dimensionlessparameter fT in FIG. 5. The loss will be less than 1 dB of the maximumintegration time is less than about 1/4 f. If D is the maximum Doppleruncertainty, then the maximum time needed to execute a serial search is2 DT². For example, if D=100 kHz, and T=1/50, then the search time mightbe as great as 80 seconds.

If timing and Doppler are jointly estimated using a serial search, thetotal acquisition time is given by 2 Df_(c) T⁴. Combining the twoexamples above, we have a total acquisition time of almost nine hours|Clearly, this is not acceptable for most applications.

Unfortunately, the requirement of tracking a user within a buildingrequires increased processing gain, which in turn requires resolution ofthe frequency to within a few Hertz, which in a spread spectrum systemcauses the acquisition time to be extremely long.

One example, of such a system is the Global Positioning System (GPS).When cold starting a GPS receiver, the receiver can take several minutesto acquire the incoming signal.

The present invention is therefore directed to the problem of developinga geopositioning system that is capable of operating at extremely lowsignal levels, such as those that might be encountered inside of abuilding, while simultaneously being capable of rapidly acquiring thesignal in the presence of large Doppler uncertainties associated withLow Earth Orbiting (LEO) satellite systems.

SUMMARY OF THE INVENTION

The present invention solves this problem by using the synchronizationwaveform rather than the data waveform for rapid acquisition of Dopplerand code timing, and by transmitting a signal from the user terminal ata frequency that is proportional to the incoming frequency, therebyeliminating the absolute oscillator frequency as a source of error. Thisenables use of a more inexpensive oscillator, which broadens the numberof practical applications of a geopositioning system.

One important aspect of the present invention is the use of asynchronization signal with a short repetition interval. The receiverintegrates over short time periods initially to produce a sequence ofintegrator outputs. These integrator outputs are then processed by aFast Fourier Transform (FFT) algorithm to determine the Dopplerfrequency close enough for the receiver to operate. This estimate is notintended to provide the accuracy needed for precise positioning; that isprovided by subsequent processing. However, this estimate is sufficientto enable the receiver to then quickly acquire the data waveform. Inthis implementation, the FFT acts, in effect, as a bank of parallelreceivers, each tuned to a different part of the spectrum. The signalacquisition process is accelerated because part of the frequency searchis performed by the FFT is parallel.

According to one aspect of the present invention, a method for receivinga signal, which includes a synchronization signal and a data signal,includes the steps of: a) integrating a synchronization signal with ashort repetition interval over short time periods to produce a sequenceof integrator outputs; b) processing the integrator outputs with a FastFourier Transform algorithm to determine a Doppler frequency that issufficiently close for the receiver to operate; and c) using the Dopplerfrequency in subsequent receiver processing to receive the data signal.

There are at least three ways to implement the above method of thepresent invention. The first way, termed the serial single correlatorimplementation, further includes the steps of: d) mixing thesynchronization signal with a synchronization code generated by a codegenerator; e) decimating the mixed synchronization signal andsynchronization code to a length of a code used to create thesynchronization signal; f) delaying the decimated mixed synchronizationsignal and synchronization code with a plurality of delay elements tocreate a plurality of signals spaced at a code interval of thesynchronization signal; g) transforming the plurality of signals fromstep f) to a plurality of frequency related signals; and h) advancing atiming of the code generator until one of the plurality of frequencyrelated signals in step g) exceeds a predetermined level.

A second way, termed the parallel matched filter implementation,includes the additional steps of: d) inputting the synchronizationsignal to a matched filter, which matched filter is matched to a codesequence used to create the synchronization signal; e) delaying anoutput of the matched filter with a plurality of delay elements tocreate a plurality of signals spaced at a code interval of thesynchronization signal; f) transforming the plurality of signals fromstep f) to a plurality of frequency related signals; and g) performingsteps d) through f) until one of the plurality of frequency relatedsignals in step f) exceeds a predetermined level.

A third way, termed the hybrid multiple correlator implementation,includes the additional steps of: d) inputting the synchronizationsignal to a plurality of mixers; e) inputting a same synchronizationcode generated to each of the plurality of mixers, but offsetting eachsynchronization code with a different time offset; f) decimating anoutput of each of the plurality of mixers to a length of a code used tocreate the synchronization signal; g) delaying each of the decimatedoutputs of the mixers with a plurality of delay elements to create aplurality of groups of delayed signals, wherein the delayed signalswithin a group are spaced apart by a code interval of thesynchronization signal; h) transforming each group of delayed signalsfrom step f) to a group of frequency related signals, thereby forming aplurality of groups of frequency related signals; and i) performingsteps d) through h) until one of the frequency related signals in theplurality of groups of frequency related signals exceeds a predeterminedthreshold.

According to another aspect of the present invention, in a communicationsystem, a method for rapidly acquiring a spread-spectrum signal at verylow signal levels with large frequency uncertainty, includes the stepsof: a) receiving a synchronization signal and a data signal; b)inputting the synchronization signal to a correlator; c) inputting acode sequence used to generate the synchronization signal to thecorrelator; d) providing a sequence of correlator outputs to a fastFourier transform, which outputs a plurality of frequency relatedsignals; e) searching each of the plurality of frequency related signalsfor a maximum amplitude; f) comparing the maximum amplitude against apredetermined threshold; and g) advancing a timing of the code sequenceand repeating steps c) through f) until the maximum amplitude in f)exceeds the predetermined threshold, which indicates that a correct codesequence is a current code sequence being used in step c) at a Dopplerfrequency determined by a frequency related to the maximum fast Fouriertransform output determined in step e).

In this method, it is particularly advantageous if the method alsoincludes the steps of: h) mixing the synchronization signal with thecode sequence; i) decimating the mixed synchronization signal and codesequence by a length of the code sequence; and j) delaying the decimatedand mixed synchronization signal and code sequence to create thesequence of correlator outputs.

In a communication system, another aspect of the present invention is amethod for rapidly acquiring a spread-spectrum signal at very low signallevels with large frequency uncertainty, which method includes the stepsof: a) receiving a synchronization signal and a data signal; b)inputting the synchronization signal to a matched filter that is matchedto the code sequence used to create the synchronization signal; c)providing a sequence of matched filter outputs to a fast Fouriertransform, which outputs a plurality of frequency related signals; d)searching each of the plurality of frequency related signals for amaximum amplitude; e) comparing the maximum amplitude against apredetermined threshold; and f) performing steps b) through e) until themaximum amplitude in d) exceeds the predetermined threshold, whichindicates that a Doppler frequency is determined by a frequency relatedto the maximum fast Fourier transform output determined in step d).

According to another aspect of the present invention, an apparatus forreceiving a signal, which includes a synchronization signal having ashort repetition interval and a data signal, includes an integrator, atransformer, and a receiver. The integrator integrates thesynchronization signal over short time periods to produce a sequence ofintegrator outputs. The transformer is coupled to the integrator andtransforms the integrator outputs to a plurality of frequency relatedsignals to determine a Doppler frequency that is sufficiently close forthe receiver to operate. The receiver is coupled to the transformer anduses the Doppler frequency in subsequent processing to receive the datasignal.

According to the present invention, one particularly advantageousembodiment of the above apparatus further includes a code generator, amixer, and several delay elements. The code generator generates a codesequence, and has a timing. The mixer is coupled to the code generatorand mixes the synchronization signal with a synchronization codegenerated by the code generator. The integrator is coupled to the mixerand decimates the mixed synchronization signal and synchronization codeto a length of a code used to create the synchronization signal. Thedelay elements are coupled to the integrator and delay the decimatedmixed synchronization signal and synchronization code to create signalsspaced at a code interval of the synchronization signal. The timing ofthe code generator is advanced until one of the frequency relatedsignals output by the transformer exceeds a predetermined level.

Another embodiment of the above apparatus of the present invention alsoincludes a matched filter, several delay elements, and a processor. Thematched filter receives the synchronization signal, and is matched to acode sequence used to create the synchronization signal. The delayelements are coupled to the matched filter and delay an output of thematched filter to create signals spaced at a code interval of thesynchronization signal. The processor monitors the frequency relatedsignals to detect when one of them exceeds a predetermined level.

Another aspect of the present invention includes an apparatus forreceiving a signal, which includes a synchronization signal having ashort repetition interval and a data signal. This apparatus includesmultiple code generators, mixers, integrators, groups of delay elementsand transformers. Each of the code generators generates the same codesequence but offset by different time offset. Each of the mixers iscoupled to one of the code generators, and mixes the code sequence withthe received synchronization signal. Each of the integrators is coupledto one of the mixers, and decimates the output of the mixer to a lengthof a code used to create the synchronization signal. Each of the groupsof delay elements is coupled to one of the integrators, and delays eachof the decimated outputs of the mixers to create a group of delayedsignals. The delayed signals are spaced apart by a code interval of thesynchronization signal. Each of the transformers transforms one group ofdelayed signals to a group of frequency related signals, thereby forminga plurality of groups of frequency related signals. The detectormonitors the plurality of groups of frequency related signals until oneof the frequency related signals in the plurality of groups of frequencyrelated signals exceeds a predetermined threshold.

According to the present invention, in a geopositioning system includinga ground station, at least one satellite, and a transceiver on thesurface of the earth, a method for determining a position of thetransceiver includes the steps of using a signal transmitted from asatellite to query the transceiver causing the transceiver to transmit aresponse, and transmitting the response from the transceiver to thesatellite using a frequency that is proportional to the incomingfrequency. This avoids the introduction of additional frequencyambiguity due to a frequency source in the transceiver.

One advantageous embodiment of the method of the present invention usesa spread spectrum signal as the signal transmitted from the transceiver.In this case, this embodiment also uses the method for rapidly acquiringthe spread spectrum signal described above.

According to the present invention, a method for determining a positionof a transceiver on the surface of the earth comprises the steps of: a)transmitting a signal from the transceiver in response to a query from asignal from a satellite; b) transmitting the response after a preciselycontrolled time interval after the transceiver receives the query; c)estimating a length of a propagation path from the satellite to thetransceiver from a time delay in the response; d) measuring a Dopplershift in the response from the transceiver; e) estimating either a firstor second derivative of a path length from the satellite to thetransceiver from the measured Doppler shift; f) estimating the satelliteposition and velocity from satellite telemetry data; g) determining anangle between the direction of satellite motion and a line of bearing tothe transceiver from either the first or second derivative and thesatellite position and velocity; and h) determining a position of thetransceiver on the surface of the earth as being one of two points wherethe surface of the earth intersects with a base of a cone defined by theangle in step g) and the estimated path length.

In this embodiment of the present invention, the angle of arrival θ(t)is determined according to the following equation: ##EQU2## where νrepresents the satellite velocity, d represents the path length (i.e.,the distance from the satellite to the terminal), and drepresents thefirst derivative of the path length.

Alternatively, the angle of arrival θ(t) can be determined according tothe following equation: ##EQU3## where ν represents the satellitevelocity, d represents the path length, and d represents the secondderivative of the path length.

In this embodiment, the remaining ambiguity concerning on which of twopoints that the surface of the earth intersects with the base of thecone the transceiver is located can be resolved by determining fromwhich satellite beam on the satellite received the response from thetransceiver. Alternatively, this ambiguity can be resolved by comparingearlier positions of the transceiver.

A transmitter for use in a geopositioning system, includes asynchronization spreading code generator, a data spreading codegenerator, a forward error corrector, a first modulator, a first chipfilter, a second chip filter, a delay element coupled to the first chipfilter, a second modulator being coupled to the delay element, a thirdmodulator, and a summer. The synchronization spreading code generatorgenerates a synchronization code spreading signal. The data spreadingcode generator generates a data code spreading signal. The forward errorcorrector encodes user data for later error correction at a receive end.The first modulator is coupled to the data code generator and theforward error corrector and modulates the encoded data on the data codespreading signal to form a data signal. The first chip filter is coupledto first modulator and filters the data signal. The second chip filteris coupled to the synchronization code generator and filters thesynchronization code spreading signal. The delay element is coupled tothe first chip filter. The second modulator is coupled to the delayelement. The third modulator is coupled to the second chip filter and is90° out of phase relative to the second modulator. The summer is coupledto the second modulator and the third modulator and outputs a combinedsynchronization and data signal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a block diagram of the geopositioning system 10 of thepresent invention.

FIG. 2 depicts the relationship between position of the user terminalrelative to the satellite position and the measurements of the pathlength and its first and second derivatives according to the presentinvention.

FIG. 3 depicts a block diagram of the timing in the user terminalaccording to the present invention.

FIG. 4 depicts a block diagram of the channel structure of thetransmitter according to the present invention.

FIG. 5 depicts the integration loss as a function of uncompensatedDoppler.

FIG. 6 depicts a block diagram of the use of the synchronization signalto rapidly acquire Doppler according to the present invention.

FIG. 7 depicts a block diagram of the acquisition process, whichinitializes the frequency and the time tracking according to the presentinvention.

FIG. 8 depicts the Early-Late correlators used in the present invention,which provide a method for estimating timing error.

FIG. 9 depicts an example of a function f(t) that is used to correct forchip waveforms that produce more rounded outputs than those depicted inFIG. 8.

FIG. 10 depicts one embodiment of the synchronization technique of thepresent invention.

FIG. 11 depicts another embodiment of the synchronization technique ofthe present invention.

FIG. 12 depicts yet a third embodiment of the synchronization techniqueof the present invention.

DETAILED DESCRIPTION

The present invention uses a Low Earth Orbiting (LEO) satellite systemto provide location and data communications services to mobile usersequipped with a receiver/transmitter. In some applications, however, theuser may not be the "customer" of the location data, such as when themobile user is a parolee, in which case the "customer" of the locationdata is the parole officer, or when the mobile "user" is a package, inwhich case the customer of the location data is the sender, deliveryservice or recipient of the package. The receiver/transmitter acts as atransponder that responds to a query transmitted over the satellitenetwork as shown in FIG. 1. The response is sent after a preciselycontrolled time interval after the transponder receives the query sothat the ground station can estimate the length of the propagation pathfrom the satellite to the transponder.

Referring to FIG. 1, the ground station 11 transmits a signal s₁ (t,f)at a time t and at a frequency f to a satellite 12, which broadcasts thesignal back to the earth. The signal s₁ (t,f) is received by the userterminal 13 as s₁ (t-τ,f+δ), i.e., at a time t-τ and at a frequency f+δ,in which τ is due to the path delay and δ is due to the Doppler shift.The user terminal 13 transmits a signal s₂ (t-τ-Δt, K(f+δ)) at a timet-τ-Δt, and at a frequency proportional to the incoming frequencyk(f+δ). This signal is received at the ground station 11 with additionalpath delay and Doppler shift, i.e., s₂ (t-2τ-Δt, k(f+δ)+δ). The userreceiver/transmitter 13 acts as a transponder so that the length of thepropagation path, the derivative of the length, and the secondderivative of the path length can be estimated by the ground station 11according to the present invention. The satellite may translate theuplink and downlink frequencies, but the frequency shift is known inadvance and is compensated for in the ground station's calculations. Inthis embodiment of the present invention, the model is simplified byassuming that no such translation takes place, however, the fundamentalapplication of the present invention remains unaffected by thisassumption.

The transponder transmits the response at a frequency that isproportional to the frequency of the received query so that the groundstation 11 can estimate the first and second derivatives of the lengthof the propagation path according to the measured Doppler shift. Thereason for the proportional response is to eliminate the absolutefrequency of the local oscillator in the transponder as a source of biasin the transmitted frequency. In addition to simplifying the analysisand improving the accuracy of the position estimate, the proportionalresponse eliminates the need for an expensive reference oscillator,which reduces the cost of the mobile unit, thereby making thegeopositioning system practical for a wide variety of uses that wereheretofore impractical from a cost standpoint.

The ground station 11 also estimates the satellite positioning usingtelemetry from the satellite 12 obtained from an on-board GPS receiver(not shown). The position of the user terminal 13 relative to thesatellite position is then determined from the path length measurementsas shown in FIG. 2. Given the satellite position p and satellitevelocity σ at a particular instant in time, the measured path lengthd(t) from the satellite to the transponder, and its first (d) and secondderivatives (d) are used to determine the angle θ(t) (at time t) betweenthe direction of satellite R(t) motion and the line of bearing d(t) tothe user terminal 13. This angle θ(t) defines a cone with the satellite12 at the origin. The user terminal position is somewhere on the circledefined by the cone and the estimated path length d. The intersection ofthis circle with the surface of the Earth yields two possible userpositions. The ambiguity can be resolved using knowledge of thesatellite beam employed to query the user terminal or by comparing thetwo possible positions with earlier position fixes.

Generally, there are three ways to resolve the ambiguity. The firstmethod determines which one of the two possible positions lies within inthe antenna beam used by the transponder. If only one position satisfiesthis condition, then this position is the correct position. If bothpossible positions lie in the beam, then other methods must be used.

The second method compares the pair of possible positions with anotherpair determined previously. The "correct" points will be close together,whereas the "incorrect" points will be further apart due to thedifferent satellite positions at the times when the pairs werecollected.

The third method uses nearby satellites to receive the transpondersignal. These satellites can also be used to locate the transponder; the"correct" locations from all satellites should align closely, whereas"incorrect" position fixes should be scattered. It may not be necessaryto locate the transponder from these satellites; once again, the beamwhich the signal is received in is associated with a known "footprint"on the surface of the Earth, and a point can be ruled out as a potentiallocation if it could not have been received on any of the satellitebeams where the signal was detected.

FIG. 2 depicts the relationship between the position of the userterminal relative to the satellite position and the measurements of thepath length and its first and second derivatives according to thepresent invention. The satellite 12, which is at a distance d(t) that isa function of time, moves at velocity σ at an angle θ(t) (also afunction of time) relative to the user terminal 13. L represents theperpendicular distance from the vector σ to the user terminal 13. R(t)represents the distance from the satellite to the point at which thevector σ intersects the line L, i.e., where θ(t) becomes 90°.

Using the Doppler method, the angle θ(t) can be calculated from thefollowing equation: ##EQU4## where ν represents the satellite velocity,d represents the path length, and drepresents the first derivative ofthe path length.

Using the Doppler slope method, the angle θ(t) can be calculated fromthe following equation: ##EQU5## where ν represents the satellitevelocity, d represents the path length, and d represents the secondderivative of the path length.

Turning to FIG. 3, the user terminal 13 uses a single frequencyreference to provide timing for the receive and transmit frequencysynthesizers and for the analog-to digital (A/D) and digital-to-analog(D/A) converters. The frequency tracking algorithm shifts the basebandfrequency of the response by a factor k so that the output frequency isrelated to the frequency of the received signal by the same factor k.This eliminates the absolute frequency of the reference as a source oferror and allows the use of less expensive oscillators in the userterminal 13.

As depicted in FIG. 3, the incoming frequency f_(rx) is downconverted bythe downconverter 31 to a new frequency f_(rx) -Mf_(ref). Thedownconverted frequency is then input to an analog-to-digital (A/D)converter 32, which also uses f_(ref) as a frequency source. The outputfrequency is then, ##EQU6## which is then input to the frequency tracker33. The output frequency from the frequency tracker 33 is proportionalto the frequency input to the frequency tracker 33, i.e., ##EQU7## Thisfrequency is then input to digital-to-analog (D/A) converter 34, whichuses the same frequency reference as the A/D converter, f_(ref). Theoutput from the D/A/ converter 34 is k·(f_(rx) -Mf_(ref)), which isinput to the transmitter upconverter 35, which upconverts the frequencyto a frequency that is proportional to the incoming frequency only,i.e., it removes the -k·Mf_(ref) term, thus transmitting at a frequencyof k·f_(rx). Consequently, the transmission frequency is directlyproportional to the incoming frequency, thus eliminating any source oferror in the frequency measurement due to inaccuracy in the referenceoscillator in the user terminal 13, and enabling the use of a lessexpensive frequency reference.

Waveform Specification

On possible embodiment of the channel structure 40 is illustrated inFIG. 4. Two baseband spread-spectrum signals 41, 43 modulate a carrier48a, 48b, a synchronization signal and a data signal 42. Thesynchronization signal modulates the in-phase (I) component of thecarrier, and the data signal 42 modulates the quadrature-phase (Q)component of the carrier. The data signal 42 is delayed one-half chip by1/2 PN chip delay 47 with respect to the synchronization signal. Thesynchronization 41 and data spreading code generators 43 producerepeated pairs of 64 chip sequences at a chip rate of 1.2288 Megahertz(MHZ). The data spreading sequence is modulated by a supercode at thedata spreading sequence repetition rate of 19200 Hz. This supercode isderived from the operation of the rate r=1/384 Forward Error Correction(FEC) coder 44 on a 50 bits per second data sequence. The modulated dataand synchronization sequences are filtered prior to carrier modulationby chip filters 46a, 46b. The outputs of the modulators are summed atsummer 49.

Synchronization Spreading Sequence

The synchronization spreading sequence is a 64-chip augmented maximallength Pseudo random (PN) code. A 63 chip sequence is augmented byadding a 0 to the sequence of five consecutive 0's.

Data Spreading Sequence

The data spreading sequence is a 64-chip augmented maximal length PNcode. A 63 chip sequence is augmented by adding a 0 to the sequence offive consecutive 0's.

Forward Error Correction

The Forward Error Correction (FEC) coder 44 produces the supercodesequence at a rate of 19200 code bits per second from a data stream of50 bits per second. The FEC coder 44 is made up of two cascaded coders,a rate 1/3 convolutional coder and a 16-ary orthogonal block coder. Theoutput of the FEC coder 44 is fed to the modulator 45 and mixed with theoutput from the data code generator 43.

Convolutional Coder

The rate 1/3 convolutional coder accepts input data at a 50 Hz rate andproduces code bits at a 150 Hz rate. The generator functions for thiscode shall be denoted as g₀, g₁, and g₂. The code output order is {c₀,c₁, c₂ } where generator output c₁ is associated with generator functiong₁. The state of the convolutional encoder, upon initialization, shallbe the all-zero state. The first code symbol output after initializationshall be the code symbol encoded with generator function g₀.

Orthogonal Block Coder

The supercode is produced by 16-ary orthogonal modulation of theconvolutional coder output. One of 16 possible modulation symbols istransmitted for each group of four code symbols. The modulation symbolshall be one of 16 mutually orthogonal sequences of 512 supercode chips.Each supercode ship corresponds in length to a single repetition of thedata spreading sequence.

Chip Filters

The chip filters 46a, 46b each accept digital data at a 1.2288 MHZ rateand produce interpolated and filtered baseband data at 4.9152 MHZ, arate four times higher.

Data Modulation

The structure of the data packets is different for the forward andreverse links. On the forward link, transmission is continuous andpackets follow each other without interruption. On the reverse link,individual units respond in a polled TDMA fashion.

Rapid Acquisition

Rapid acquisition of the spread-spectrum signal is important for tworeasons. In the user terminal, minimizing acquisition time extends thelife of the battery. In the ground station, minimizing acquisition timeexpands the capacity of the channel, allowing a greater number of usersor more data. The synchronization signal that is transmitted inquadrature with the data signal enables rapid acquisition.

FIG. 10 depicts one possible embodiment 100 for performingsynchronization according to the present invention. The input signal ismixed with the codeword sequence output by the code generator 101. Theresulting output from the mixer 102 is coupled to an integrator 103,which decimates the input by the length of the code sequence. Theintegrator contents is then dumped to a series of delay elements 104,the outputs of which are input to a Fast Fourier Transform 105, whichprovides multiple outputs. The delay elements 104 provide FourierTransform inputs spaced at the code interval. The code generator timingis slowly advanced until a signal is detected at one of the FourierTransform outputs.

To overcome the two problems of acquisition of timing and Doppler,parallel search techniques can be used to overcome the limitations ofserial acquisition. By replacing the correlation receiver (i.e., thecode generator 101, the mixer 102, and the integrator 103, with amatched filter 111,) the designer can eliminate the timing acquisitionentirely. In this case, the matched filter 111 is matched to the codesequence. As before, operation continues until a signal is detected atone of the Fourier Transform outputs. This embodiment 110 is depicted inFIG. 11. The drawback is the computation rate Tf_(c) ² might requireexpensive hardware to operate. With T=1/50 seconds, and f_(c) =1 MHZ,the required rate is 20 billion operations per second.

Doppler acquisition can be expedited by operating a bank of correlatorsin parallel. A bank of N such correlators reduces the maximum searchtime by a factor of N. This embodiment is depicted in FIG. 12. Eachcorrelator is as shown in FIG. 10, but the code generator 101 in eachhas a different time offset than the others. In this embodiment,operation continues until a signal is detected at one of the FourierTransform outputs of any correlator channel.

The present invention uses the synchronization signal to resolve Dopplerambiguity as shown in FIG. 6. The bank of correlators 61 f(z) operatewith the L=64 chip synchronization code sequence. The correlator outputsare decimated to 19.2 kHz and then processed by a Fast Fourier Transform(FFT) 64. A total of 512 parallel correlator outputs provides a totalprocessing gain of 64*512=32768. The FFT output is searched for themaximum amplitude. If this amplitude exceeds a threshold, then thesignal is assumed to have been detected at the current correlator codeoffset with a Doppler frequency given by the frequency of the maximumFFT output. If no signal has been detected, then the correlator codeoffset is incremented and the entire procedure is repeated until allpossible 64 code offsets have been tested. Multiple correlators can beused to speed this procedure.

After the synchronization and data code correlators are properlysynchronized to the signal, symbol synchronization is performed. Thedata correlator outputs are further processed by a 512-tap matchedfilter to search for a frame synchronization symbol that begins eachtransmission. When such a symbol is detected, subsequent symbols areprocessed by a Forward Error Correction (FEC) algorithm to recover thedata.

Tracking

Once the initial Doppler offset and code offset have been determined,time and frequency tracking loops refine the estimates and adapt tochanging Doppler induced by satellite motion as shown in FIG. 7.

The time tracking loop uses a pair of correlators operating at slightlydifferent delays to measure the timing error as shown in FIG. 8. Theidealized triangular correlator output is shown for the case ofrectangular chips. Other chip waveforms will produce more roundedoutputs. An example of the function f(t) that is used to correct forthis rounding is shown in FIG. 9.

This section describes how the user terminal operates. The groundstation is free to use other non-real-time techniques for extractingtiming.

Additional Positional Accuracy Technique

The positional accuracy of the system depends upon the relativepositions of the satellite and the transponder. There are somegeometries where the positioning accuracy is degraded. One way toimprove the positioning accuracy under these circumstances is to use thesignals received from other satellites. For example, if the signal istransmitted over the primary satellite at time t₁ and is received attime t₂, then the propagation time from the ground station to thetransponder through the primary satellite is t_(p) =(t₂ -t₁)/2-t₀, wheret₀ is the transponder time delay. If the transponder time response isalso received over a secondary satellite at time t₃, then thepropagation time from the ground station to the transponder through thesecondary satellite is t_(s) =t₃ -t₁ -t_(p) -t₀. This propagation timetaken together with knowledge of the position of the secondarysatellite, defines a sphere around the secondary satellite thatintersects the Earth along a curve containing the transponder location.This curve can be combined with the information derived from the primarysatellite to provide a more accurate estimate of the transponderlocation. This approach can be extended to the case of multiplesatellites and to also exploit the Doppler and Doppler derivativeinformation derived from multiple satellites.

What is claimed is:
 1. A method for determining a position of atransceiver on the surface of the earth comprising the steps of:a)transmitting a signal from the transceiver in response to a query from asignal from a satellite; b) transmitting the response after a preciselycontrolled time interval after the transceiver receives the query; c)estimating a length of a propagation path from the satellite to thetransceiver from a time delay in the response; d) measuring a Dopplershift in the response from the transceiver; e) estimating a firstderivative of a path length from the satellite to the transceiver fromthe measured Doppler shift; f) estimating the satellite position andvelocity from satellite telemetry data; g) determining an angle betweenthe direction of satellite motion and a line of bearing to thetransceiver from the first derivative and the satellite position andvelocity; and h) determining a position of the transceiver on thesurface of the earth as being one of two points where the surface of theearth intersects with a base of a cone defined by the angle in step g)and the estimated path length.
 2. The method according to claim 1,wherein the step g) further comprises determining an angle of arrivalθ(t) according to the following equation: ##EQU8## where σ representsthe satellite velocity and drepresents the first derivative of the pathlength.
 3. The method according to claim 1, further comprising the stepof:i) determining on which of the two points that the surface of theearth intersects with the base of the cone from step h) the transceiveris located from which satellite beam on the satellite received theresponse from the transceiver.
 4. The method according to claim 1,further comprising the step of:i) determining on which of the two pointsthat the surface of the earth intersects with the base of the cone fromstep h) the transceiver is located by comparing earlier positions of thetransceiver.
 5. The method according to claim 1, further comprising thestep of:i) transmitting the response from the transceiver to thesatellite using a frequency that is proportional to the incomingfrequency.
 6. A method for determining a position of a transceiver onthe surface of the earth comprising the steps of:a) transmitting asignal from the transceiver in response to a query from a signal from asatellite; b) transmitting the response after a precisely controlledtime interval after the transceiver receives the query; c) estimating alength of a propagation path from the satellite to the transceiver froma time delay in the response; d) measuring a Doppler shift in theresponse from the transceiver; e) estimating a second derivative of thepath length from the satellite to the transceiver from the measuredDoppler shift; f) estimating the satellite position and velocity fromsatellite telemetry data; g) determining an angle between the directionof satellite motion and a line of bearing to the transceiver from thesecond derivative and the satellite position and velocity; and h)determining a position of the transceiver on the surface of the earth asbeing one of two points where the surface of the earth intersects with abase of a cone defined by the angle in step g) and the estimated pathlength.
 7. The method according to claim 6, wherein the step g) furthercomprises determining an angle of arrival θ(t) according to thefollowing equation: ##EQU9## where σ represents the satellite velocity,d represents the path length, and d represents the second derivative ofthe path length.
 8. The method according to claim 6, further comprisingthe step of:i) determining on which of the two points that the surfaceof the earth intersects with the base of the cone from step h) thetransceiver is located from which satellite beam on the satellitereceived the response from the transceiver.
 9. The method according toclaim 6, further comprising the step of:i) determining on which of thetwo points that the surface of the earth intersects with the base of thecone from step h) the transceiver is located by comparing earlierpositions of the transceiver.
 10. The method according to claim 6,further comprising the step of:i) transmitting the response from thetransceiver to the satellite using a frequency that is proportional tothe incoming frequency.